Biologists with West Fork Environmental collect sockeye carcasses as they cover more than 20 miles of salmon habitat in the Cedar River each week.

So far this season, they've picked up 1,600 dead fish. They mark the GPS location of each carcass and tag the fish. This year, they've found more sockeye dying before they ever spawn.

"That's been one of the biggest Eureka’s and surprises, but it's not a good one, because there's mystery around this disease or suite of diseases, and then problems with what we can do to counteract it," Michelle Koehler, who works with Seattle Public Utilities on the carcass survey.

The heads of the salmon are dissected for the otolith, which is a piece of the inner ear that tells scientists whether the fish are hatchery or wild sockeye. It also tells age and provides clues about environmental conditions during the sockeye's lifetime.

Scientists want to know how hatchery sockeye affect wild salmon. The collection also shows whether the hatchery fish are spawning around all parts of the river.

"That's what these scientists have actually been able to verify," said Michelle Koehler, who works with Seattle Public Utilities on the carcass survey.

The disease that is killing the salmon is likely caused by a parasite, but it could be more common with warming water temperatures. There is no human health risk associated with the disease, according to West Fork Environmental.